The combined use of a partial response linear waveform equalizer circuit and Viterbi decoding is effective to decode a reproduction signal detected from an optical disk properly. This is indicated in, for example, High-Definition DVD (HD DVD)-ROM Part 1 (physical specification) published by DVD Forum. A disk control circuit in an optical disk player compatible with the standard rotates an optical disk at a predetermined rotation speed. An optical pickup reads a reproduction signal recorded on the optical disk. The reproduction signal is amplified by a preamplifier and amplified by an AGC circuit to a predetermined amplitude.
Further, the reproduction signal is subject to A/D conversion, subject to waveform equalization by a linear waveform equalizer circuit, and decoded by Viterbi decoding. As a result, image data and music data recorded on the optical disk are reproduced. As the recording density of an optical disk is increased, nonlinear distortion in the reproduced waveform is increased so that a linear waveform equalizer circuit alone is not sufficient to reduce distortion. For this reason, a nonlinear waveform equalizer circuit is used to reduce nonlinear distortion. A neural network is used to implement a nonlinear waveform equalizer circuit (see, for example, patent document 1).    [patent document No. 1] JP 10-106158